BTRFS compression

https://btrfs.wiki.kernel.org/index.php/Compression

Compression:

• ZLIB – slower, higher compression ratio (uses zlib level 3 setting)
• LZO – faster compression and decompression than zlib, worse compression ratio, designed to be fast
• ZSTD – (since v4.14) compression comparable to zlib with higher compression/decompression speeds and different ratio levels details
  • NOTE: odler GRUB version cannot read ZSTD compressed files (from /boot)

Default compression for kernels up to 3.6 it's ZLIB. For compatibility with old kernel ZLIB is default. LZO is fast and new algo, but can be buggy.

Does a balance operation recompress files?

No. Balance moves entire file extents and does not change their contents. If you want to recompress files, use btrfs filesystem defrag with the -c option.
Balance does a defragmentation, but not on a file level rather on the block group level. It can move data from less used block groups to the remaining ones, eg. using the usage balance filter.

NOTE: Defragmentation with recompression destroys deduplication.

• Size changed from 6,94 GB to 7,39 GB
• After deduplication with 1024k block drops to 7.17 GB (net change in shared extents of: 490.0M)
• After deduplication with 4k block drops from 7.17 GB to 6,94 GB(a net change in shared extents of: 306.6M)

Can be enabled during mount time:

mount BTRFS -o remount,compress=zlib
mount BTRFS -o remount,compress-force=zlib

if the first portion of data being compressed is not smaller than the original, the compression of the file is disabled – unless the filesystem is mounted with compress-force

compress-force=<method> - Enable compression even for files that don't compress well, like videos and dd images of disks.

Test data of size 651MB takes 371MB after compression.

Also single file compression possible using chattr +c filename.txt

To recompress exisitng files, start defragmentation with -c param:

btrfs filesystem defrag -c file.iso

btrfs filesystem defrag -c -r mydir

It is not possible to get compression ratio of file.

Internals:

• In compressed extents, individual blocks are not compressed separately; rather, the compression stream spans the entire extent.
• BTRFS: There is a simple decision logic: if the first portion of data being compressed is not smaller than the original, the compression of the file is disabled – unless the filesystem is mounted with compress-force

btrfs property set <file> compression <zlib|lzo|zstd>